Positive temperature coefficient of resistance (PTCR) devices can be used for temperature sensing, heat sensing, current sensing, liquid level sensing, generating heat, regulating the temperature for other devices; and voltage clamping and current suppression to provide circuit protection for other devices.
Most PTCR devices are based on the grain boundary PTCR effect. If the bulk materials are ceramics such as barium titanate based ferroelectric semiconductor material, the devices are fabricated by standard solid state reaction methods, with the powders cold-pressed and sintered at high temperatures. Usually, the ceramic devices have additives such as Sr, Zr, Ca, Pb to control the Curie point; Y, Sb to impart the semiconducting properties; with Fe, Cu, and Mn, to enhance the bulk PTCR effect.
The disadvantage of a PTCR device based on the grain boundary PTCR effect is that the device is bulky and difficult to integrate with other electronic devices into a monolithic form.
It is desirable to provide a new method for making a device wherein the PTCR effect is at electrode level, and which can be easily integrated into other electronic devices for various applications.
U.S. Pat. No. 4,895,812, "Method of Making Ohmic Contact to Ferroelectric Semiconductors", teaches a method for making ohmic contacts to ferroelectric semiconductors. The patent teaches that an electrode material, which can be any electronically conductive material as long as it is thermal-chemically and thermal-mechanically stable with the semiconducting substrate material, is layered on the substrate. The layer is heated to a temperature higher than the Curie point. Upon cooling, the resulting electrode is ohmic to the ferroelectric semiconductor, as the electrode resistance is lower than the bulk resistance. No mention or suggestion is made of a PTCR effect.